Cochlear implants and other inner ear prostheses are one option to help profoundly deaf or severely hearing impaired persons. Unlike conventional hearing aids that just apply an amplified and modified sound signal, a cochlear implant is based on direct electrical stimulation of the acoustic nerve. Typically, a cochlear implant stimulates neural structures in the inner ear electrically in such a way that hearing impressions most similar to normal hearing are obtained.
More particularly, a normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the eardrum 102, which moves the bones of the middle ear 103, which in turn excites the cochlea 104. The cochlea 104 includes an upper channel known as the scala vestibuli 105 and a lower channel known as the scala tympani 106, which are connected by the cochlear duct 107. In response to received sounds transmitted by the middle ear 103, the fluid filled scala vestibuli 105 and scala tympani 106 function as a transducer to transmit waves to generate electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain.
Some persons have partial or full loss of normal sensorineural hearing. Cochlear implant systems have been developed to overcome this by directly stimulating the user's cochlea 104. A typical cochlear prosthesis essentially includes two parts: the speech processor and the implanted stimulator 108. The speech processor (not shown in FIG. 1) typically includes a microphone, a power supply (batteries) for the overall system and a processor that is used to perform signal processing of the acoustic signal to extract the stimulation parameters. In state-of-the art prostheses, the speech processor is a behind-the-ear (BTE-) device. The stimulator generates the stimulation patterns and conducts them to the nerve tissue by means of an electrode array 110 which usually is positioned in the scala tympani in the inner ear. The connection between speech processor and stimulator is usually established by means of a radio frequency (RF-) link. Note that via the RF-link both stimulation energy and stimulation information are conveyed. Typically, digital data transfer protocols employing bit rates of some hundreds of kBit/s are used.
One example of a standard stimulation strategy for cochlear implants is called “Continuous-Interleaved-Sampling strategy” (CIS), which was developed by B. Wilson (see, for example, Wilson B S, Finley C C, Lawson D T, Wolford R D, Eddington D K, Rabinowitz W M, “Better speech recognition with cochlear implants,” Nature, vol. 352, 236-238, July 1991, incorporated herein by reference in its entirety).
The overall power budget of a contemporary cochlear prosthesis using an RF-link is essentially described by
                                          P            BATT                    =                                    P              SIG                        +                                          P                STIM                            η                                      ,                            (        1        )            
where PBATT is the power delivered by the battery, PSIG is the power consumption of the (external) signal processing, PSTIM represents the power consumption of the implanted stimulator (including the actual electrical stimulation power), and η is the overall power efficiency of the RF-link. The ratio
      P    STIM    ηrepresents the power flowing into the RF-transmitter. Note that PSTIM and PSIG are first of all determined by the stimulation strategy used. For example, for CIS-strategy as described above, typical values are PSTIM=6 mW and PSIG=6 mW. Assuming η=0.25 results in PBATT=30 mW.Totally Implantable Cochlear Implant (TICI)
A totally implantable cochlear implant (TICI) is a cochlear implant system without permanently used external components. A TICI typically includes a microphone and subsequent stages perform audio signal processing for the implementation of a particular stimulation strategy (e.g., CIS). It also includes stimulation electrodes, power management electronics, and a coil for the transcutaneous transmission of RF signals.
Unlike a pacemaker implant, the power supply of a TICI generally cannot be established by means of a non-rechargeable battery. This is because the overall pulse repetition rate of a TICI is much higher. For example, typically about 20 kpulses/s are generated by a cochlear implant using CIS, as compared to about 1 pulse/s in a pacemaker. Besides, a cochlear implant typically performs complex audio signal processing, as compared to simple sensing tasks performed in a pacemaker. Consequently, a rechargeable battery is typically required in a TICI, which needs recharging after a particular time period of operation. The external device used for charging includes equipment for the transcutaneous transmission of RF signals. It may be body worn and contain a second rechargeable battery and optionally other auxiliary devices like, without limitation, remote control, and FM-equipment.
Quick Charging
Recharging of an implanted rechargeable battery is conventionally achieved by means of an inductive RF-link. The standard approach, designated as “quick charging”, involves charging up the battery as fast as possible, limited only by the maximum charging current. In typical state-of-the-art battery technologies (e.g., 3.6V Li-Ion technology), the absolute maximum charging current in mA is nominally equal to the capacity C. For example, for a battery with capacity C=20 mAh, the absolute maximum charging current is 20 mA, and thus it requires about 1 h to charge up an empty battery. However, the following aspects of quick charging need to be considered.
(a) The quick charging paradigm, i.e., long periods of slow discharge down to the lower energy limit and then a comparatively short period of recharge with maximum charging current up to the upper energy limit imposes considerable stress on the battery and might reduce numbers of charging cycles, before the battery looses its capacity. Typically only 500-1000 cycles for Li-Ion technology are obtained in such a mode. Assuming a battery capacity that is sufficient to operate the TICI for one day requires one charging session per day. For a maximum of 1000 cycles this means that after 3 years of implantation, the TICI, or at least the TICI rechargeable battery, may have to be replaced. However, a maximum period of only 3 years may be impractical for a wide variety of cochlear implant applications.
(b) The life time of the battery could be enhanced without increasing the maximum number of charging cycles by increasing the capacity. For example, if the capacity is sufficiently large to operate the TICI for 5 days instead of only one, then the maximum battery life time is also increased to about 15 years, which may be considered acceptable. However, increasing the capacity by a factor 5 also increases the volume of the capacity by the same factor, and this might be impossible with respect to the very limited space within a cochlear implant. Approaches to position an implanted rechargeable battery not in the local vicinity of the inner ear, but somewhere else in the body are technically feasible, but not currently implemented. For example, a rechargeable battery at the position of a pace maker device in the upper chest region may make sense from a technical point of view.
(c) Quick charging may increase the temperature of the battery and with it the temperature of the surrounding tissue. The amount of temperature rise can depend on many factors including magnetic strength RF-field, charging current, charging time, TICI mass, and blood circulation. A maximum temperature rise of 1K is tolerable.
(d) Assuming state-of-the-art battery technology, the maximum capacity for a rechargeable battery positioned in the local vicinity of the ear is limited to approximately tens of mAh. This limitation is due to space requirements and allows a TICI operation for about one day. However, from a patient point of view, the idea of a daily and obligatory charging session lasting for at least one or two hours often is not an attractive option.
Other Rechargeable Battery Considerations
An implanted rechargeable battery may not be appropriate in certain circumstances. For example, for very young children an implanted rechargeable battery may be too large or heavy. Various patients may not appreciate the idea of carrying a power source in the head, or the somewhat cumbersome (daily) recharging procedure.